PROJECT SUMMARY/ABSTRACT The primary aim of the Health and Retirement Study (HRS) cooperative agreement is to collect and distribute multi-disciplinary data to support research on aging. Cognitive impairment and dementia, including Alzheimer's disease and Alzheimer's disease related dementias (AD/ADRD), impose enormous burdens on families and government policies. To provide the research community with new and richer data to study the prevalence, predictors, and outcomes of cognitive impairment and dementia, we undertook the HRS Harmonized Cognitive Assessment Protocol (HCAP) Study during the HRS 2016 field period, in which we administered a supplemental in-home 1-hour battery of cognitive tests to about 3,200 randomly selected and nationally representative HRS respondents aged 65+, as well as a 20-minute informant interview. In the proposed HCAP 2020 follow-up study, we will re-administer the same HCAP in-home cognitive assessment and informant report to all surviving members of the original HCAP sample (n ~ 2,250), and to a new random sample of those aged 65 to 68 in 2020 (n ~ 720). The HCAP 2020 follow-up study will provide extensive new data to better assess trajectories of cognitive decline among older US adults, including the incidence of new cognitive impairment and dementia. These data will also provide an unprecedented opportunity to better understand trends in the incidence and prevalence of dementia in the US and around the world.